Submerged arc welding (for example, refer to Patent Literatures 1 and 2) using two or more electrodes is popularized as seam welding of large-diameter steel pipes such as UOE steel pipes, spiral steel pipes, and the like, and double one layer welding with high efficiency in which the inner side is welded in one pass and the external side is welded in one pass is widely used in view of improvement of productivity of large-diameter steel pipes.
In double one layer welding, it has the need to secure a depth of penetration for sufficiently overlapping an internal weld metal with an external weld metal so as not to produce an unmelted part, and thus welding is generally performed by supplying a large current of 1000 A or more.
On the other hand, seam welding of large-diameter steel pipes has the problem of deterioration in toughness of welded zones, particularly welded heat affected zones, and thus has the need to decrease welding heat input as much as possible in order to improve toughness of welded zones. However, a decrease in welding heat input increases the possibility of producing lack of penetration, easily produces an unmelted part, and causes the problem of easily producing surface defects such as undercut and the like.
Therefore, a welding technique is researched for both securing a depth of penetration and improving toughness of welded zones, and achieving a wide bead width and preventing surface defects such as undercut in seam welding of large-diameter steel pipes.
For example, Patent Literature 3 discloses a welding method including combination of gas metal arc welding and submerged arc welding, in which a deep depth of penetration is secured by gas metal arc welding, and then submerged arc welding with a large amount of metal deposit is performed by using two wires for one torch for the submerged arc welding, the two wires being arranged at a right angle with a weld line direction, thereby achieving a wide bead width to prevent surface defects such as undercut and the like.
However, the technique disclosed in Patent Literature 3 can exhibit the effect of widening a bead width at a weld speed exceeding 3 m/min, but at a weld speed of 3 m/min or less, the effect of widening a bead width cannot be obtained particularly in welding of a thick material having a thickness of over 20 mm. Therefore, the bead width is required to be widened by a method of increasing a welding voltage or the like, resulting in the problem of difficult in decreasing welding heat input. Further, combination of gas metal arc welding and submerged arc welding complicates an apparatus configuration and thus increases a load required for control of welding conditions and for maintenance of an apparatus.
Patent Literature 4 discloses a submerged arc welding method for high-speed welding with multiple electrodes, in which a magnetic field is applied to an unsolidified weld metal to control molten iron flow, thereby attempting to prevent surface defects such as undercut and the like.
However, the technique disclosed in Patent Literature 4 can exhibit the effect of widening a bead width but requires an apparatus introduced for applying a magnetic field and thus complicates an apparatus configuration and increases a load required for control of welding conditions and magnetic field setting conditions and for maintenance of an apparatus.